WASHINGTON (AP) — U.S. long-term mortgage rates continued to fall this week, breaching already historically low levels and offering an incentive to potential homebuyers.

Mortgage buyer Freddie Mac said Thursday the average rate for a 30-year fixed-rate mortgage dropped to 3.51% from 3.60% last week. The benchmark rate stood at 4.46% a year ago.

The average rate on a 15-year mortgage declined to 3% from 3.04% last week.

Federal Reserve policymakers continued to hold interest rates low at their latest meeting this week. But the Fed chair warned that the viral outbreak in China poses a new threat to the strengthening global economy.

Fed Chairman Jerome Powell said Wednesday that the signing of a preliminary U.S.-China trade deal earlier this month, the resolution of Brexit and continuing low interest rates in the U.S. and abroad had suggested the world economy would start to expand more quickly after being held back by trade conflicts. That scenario is now complicated by the emergence of the deadly virus in China, Powell said at a news conference.

Freddie Mac surveys lenders nationwide between Monday and Wednesday each week to compile its mortgage rate figures. The average doesn’t include extra fees, known as points, which most borrowers must pay to get the lowest rates.

The average fee on 30-year fixed-rate mortgages fell to 0.7 point from 0.8 point last week. The average fee for the 15-year mortgage also declined to 0.7 point from 0.8 point.

The average rate for a five-year adjustable-rate mortgage decreased to 3.24% from 3.28% last week. The fee was unchanged at 0.3 point.